Tools for cutting threads into the interior surfaces of bores comprise two general types, depending on the relative diameter of the bore. With larger diameter bores, tool design is more flexible. It is possible to use a tool that includes special wear resistant cutting inserts to actually cut the threads. These inserts can be made of a material different than the base material of the tool itself. The main body of the tool is a metal bar that has the requisite toughness and machinability. Mounting an insert to the bar takes radial room, but this is readily available in large diameter bores.
Such a two part design is not possible in tools used to thread smaller diameter holes, which include taps, thread hobs and the like. These generally are made of one solid piece of material, because of space constraints. The cutting teeth are cut from the same material as the main body of the tool itself, although the whole tool may be coated. Typically, such tools are manufactured by machining axial flutes into the cylindrical main body of the tool, which creates a plurality of exposed lands. Then, a pattern of regular cutting teeth is machined across the lands, matching the thread pattern that is desired. No good method exists for tipping these much smaller cutting teeth with a secondary, more wear resistant material.